THE ANCESTRY OF THE VERTEBRATES 



659 



hypophysial ingrowth is directed towards the anterior termination of the 

 notochord. If it were to grow into the notochord, its relations would be 

 similar to those of the proboscis to its sheath. 



Hubrecht, however, fails to show that the proboscis sheath is endo- 

 dermal. It is in fact mesodermal, while the notochord is endodermal. 

 The two structures are, therefore, not comparable. This objection 

 remains, even if the notochord sometimes arises as a hollow outgrowth 

 and therefore may have been, like the proboscis sheath, originally hollow. 

 Two hollow structures may be compared, but only if derived from the 

 same germ layer. Further difficulty appears in the fact that in the 

 lowest vertebrates, the myxinoids, the hypophysis connects posteriorly 

 with the pharynx, not with the notochord. 



NERVE CORD 



Fig. 532. — Balanoglossus, the typical genus of the Hemichordates, seen in left lateral 

 aspect. The possession of both dorsal and ventral nerve-cords links hemichordates 

 on the one hand with invertebrates and on the other with vertebrates. (Redrawn after 

 Stempell.) 



Nemerteans certainly do resemble vertebrates in having paired seg- 

 mentally arranged nerves connected with the paired nerve-cord, and 

 with a dorsal nerve-cord, when this is also present. There is, however, 

 not a regular alternation of sensory and motor nerves in nemerteans as in 

 amphioxus and vertebrates. 



The case which Hubrecht is able to make in favor of the nemertean 

 ancestry of vertebrates is obviously not very convincing. This, however, 

 should not blind us to the fact that, in some ways, such as the presence of 

 gonadic sacs, the flatworms exhibit traits which may be considered 

 ancestral to those of vertebrates. 



BALANOGLOSSUS THEORY 



Bateson (1886) was the first to suggest that bilaterally segmented 

 animals have evolved in two directions. In one group, the alimentary 

 canal pierced the nervous system, leading to annelids and arthropods; 



